This invention relates generally to borehole formation evaluation instrumentation and methods of using such instrumentation in the drilling of directional wells. More particularly, this invention relates to a method for measuring the position of a drillstring while drilling a horizontal borehole and maintaining the drillstring within desired boundaries using electromagnetic propagation based earth formation evaluation tools.
Borehole formation evaluation tools are known which measure phase and/or amplitude of electromagnetic waves to determine an electrical property (such as resistivity or permitivity) of a section of a borehole. Typically, the existing tools used for this application are composed of one or more transmitting antennas spaced from one or more pairs of receiving antennas. An electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes by the two receiving antennas. In a resistivity measuring tool, magnetic dipoles are employed which operate in the mf and lower hf spectrum. In contrast, permitivity tools utilize electric dipoles in the VHF or UHF and microwave ranges.
In a known resistivity sensor of the type hereinabove discussed which is used by Teleco Oilfield Services, Inc., assignee of the present application, the resistivity sensor measures both phase difference and amplitude ratio to provide two resistivities with different depths of investigation. A signal received in a first (far) receiving antenna is shifted in phase and its amplitude will be less than the signal received in a second (near) receiving antenna. Resistivities are then derived from both the phase difference (R.sub.pd) and the amplitude ratio (R.sub.ar) of the received signals. The differential measurement is primarily responsive to the formation opposite the receiving antennas and is less sensitive to the borehole and/or variations in the transmitted signal as in prior art sensing devices. An example of a formation evaluation instrument of this type is described in FIGS. 1 and 2 of U.S. Pat. No. 5,001,675 which is assigned to the assignee hereof and fully incorporated herein by reference. The formation evaluation tool acquires the resistivity data in real time and then transmits this information to the drilling operator using any suitable measurement-while-drilling transmission technique such as mud pulse telemetry (see U.S. Pat. Nos. 3,982,431, 4,013,945 and 4,021,774) or the information is stored downhole for review after retrieval of the tool.
In drilling a horizontal well, the goal is to drill the well in such a way that the well stays within the pay zone (i.e., a selected bed or stratum) for as long as possible in order to achieve a higher recovery rate. Before drilling a horizontal well, a course of drilling is planned based on knowledge about the pay zone and the boundaries between the pay zone and its neighboring beds. Because of the uncertainties in the knowledge about the bed boundaries and the errors in directional drilling, the well being drilled often can not stay in the pay zone as desired. It is, therefore, very important to know the relative position of the drill bit and the well with respect to the bed boundary in real time so that when the well is very close to the bed boundary, proper action is taken, ensuring that the well will stay within the pay zone.